1. Field of the Invention:
The present invention relates to telephones of various types, and more particularly to a hands-free type automobile telephone and a general-purpose telephone, the hands-free type telephone being provided with an audio amplifier and classified into International Patent Classification Nos.: H04M 1/60 and 1/04.
2. Description of the Prior Art:
In recent years, automobile telephones are used extensively, and, therefore, automobiles provided with the automobile telephones increase. In use, a user of the automobile telephone holds a handset of the telephone in his hand. Consequently, when a driver uses the automobile telephone, he is forced to drive a car through one-handed operation while restricted in freedom of his head movement, which seriously influences safety driving of the car.
In order to ensure safety driving of the car, hitherto, there is employed a hands-free type telephone comprising: a handset having a transmitter at one end thereof and a receiver at the other end thereof; a cradle-shaped coupler for supporting the handset at opposite ends thereof, the cradle-shaped coupler being provided with a two-way amplifier circuit having a speaker oppositely disposed from the transmitter of the handset and a microphone oppositely disposed from the receiver of the handset; a separate microphone connected with the speaker of the cradle through an electronic circuit; and a separate speaker connected with the microphone of the cradle through the electronic circuit; whereby the telephone is used through the separate speaker and the separate microphone in hands-free condition.
In such commercially available conventional hands-free type telephone, there is no standard with respect to the handset thereof in size and mounting angle of its transmitter and receiver. Accordingly, in order to prevent a howling action or a high-pitched audio tone put out by the receiver, so as to permit the telephone to be suitably operated, which howling action is resulted from misalignment in coupling of the handset with the cradle-shaped coupler, the handset must be an exclusive one or must be selected in size and mounting angle so as to align with individual cradle-shaped couplers.
In addition, in order to prevent the howling action, it is necessary to considerably reduce audio volume and tone so as to prevent sounds put out by the speaker from entering the microphone, which makes it difficult to recognize contents of the sounds.